Process for the production of electrophotographic images



y 3, 1969 R. MENQLD 3,443,936

PROCESS FOR THE PRODUCTICN .cF ELECTROPHOTOGRAPHIC IMAGES Filed July 23. 1965 Sheet 1 or 2 3 1 FIG. 1 FIG. 3

INVENTOR. RICHARD MENOLD ATTORNEYJ R. MENOLD 3, 3,9 PROCESS FOR THE PRODUCTION OF ELECTRQPHOTQGRAPHIC IMAGES May 13, 1969 Sheet 3 012 Filed July 23, 1965 FIG. 5

FIG. 6

INVENTOR. RICHARD MENOLD (AM, xwi

N. WA

AT TORNEY.

States U.S. Cl. 96-1 11 Claims ABSTRACT OF THE DISCLOSURE Electrophotographic layer sensitized with dye has the dye very effectively bleached or changed in color by corona discharge combined with illumination by light having wave-lengths outside the region of fundamental lattice absorption by the electrophotographic material. The bleaching or color change is most effective when the illumination light is in the region of the dyes maximum absorption. When the illumination light is in the region of fundamental lattice absorption the bleaching or color change of the dye is retarded. When the illumination light has components within and without the region of fundamental lattice absorption, bleaching or color change is also retarded but takes place preferentially where there is a sharp decay in the conductivity of the electrophotographic layerthat is at the outlines of an image. By projecting an image of a screen on the sensitized electrophotographic layer along with an image to be reproduced, the sharp decay areas are made coextensive with the image areas rather than the image outlines. Positive or negative dye images are obtained by appropriately arranging the exposures to either preferentially bleach the light or dark portions of the image to be reproduced.

The invention relates to an electrophotographic process for the direct production of negative or positive images by imagewise exposure of photoconductive layers containing a dye, while the layer is simultaneously subjected to a corona discharge.

The production of direct copies upon exposure of a dyed photoconductive layer and simultaneous charging of the layer has already been disclosed in US. Patent 3,082,- 085. The color changes always occur at the light-struck areas.

However, in many cases the quality of the images produced by the above process does not meet practical requirements, particularly because very long exposure times are necessary.

The object of the invention is to provide a dry process for the production of electrophotographic images.

Another object of this invention is to provide a process which directly produces a visible copy upon exposure to the object to be reproduced and does not necessarily require a separate step of developing.

I now have found that the ability of a substance contained in a photoconductive layer to undergo a color change by exposure to light and simultaneous action of charges depends to a very great extent on whether the light employed for exposure contains wavelengths from the region of the fundamental lattice absorption of the photoconductive compound.

Under the term fundamental lattice absorption it is to be understood the optical absorption of the photoconductive solid which results from the excitation of electrons across the gap from the valence band to the conduction band. as defined for instance, by R. H. Bube Photoconductivity of Solids published by John Wiley and Sons Inc., New York-London 1960, page 211 ff. In other atent O 3,443,936 Patented May 13, 1969 words light of this range of wavelengths have sufiicient energy to excite an electron across the gap from the valence band to the conduction band. In the case of photoconductive zinc oxide, light of a wavelength of about 385 millimicrons and less which supplies to the zinc oxide the energy of at least about 3.2 ev., satisfies these requirements.

This dependency is so great that by exposure to light from the fundamental lattice absorption region, the dyes are even stabilized with regard to the changeability of their color under the conditions of the process. In other words the changeability of the color of the substances contained in the photoconductive layer can be very greatly accelerated by the simultaneous action of light and charges if the photoconductive layer is exposed only to light which does not contain wavelengths from the fundamental lattice absorption region. This discovery is extremely unexpected. Hitherto, the experts were of the opinion that the higher the energy of the light to which the dye was exposed the more rapid would be the bleaching or color change. Thus, in the known process referred to hereinbefore, the exposure with UV light is preferred.

The present invention can be utilized in many different ways for the production of negative or positive electrophotographic images. Negative images are images produced by a color change in the unexposed areas; positive images conversely are images produced by a color change in the exposed areas.

(1) PROCESS FOR THE PRODUCTION OF POSITIVE IMAGES In the simplest case, e.g., a dyed photoconductive layer is exposed to the original to be reproduced with light which does not contain any wavelenths from the fundamental lattice absorption region. In the case of photoconductive layers which contain zinc oxide as photoconductor this would, for example, be light from the visible or infrared region of the spectrum. The dye is bleached at the lightstruck areas and a positive image of the original is obtained.

If light used for the exposure contains only rays with wavelengths from the fundamental lattice absorption region, i.e., ultra violet light in the case of Zinc oxide photoconductor, no changes in the light-struck areas are obtained in the usual exposure times. Only after prolonged exposure a slight bleaching of the unexposed areas occurs.

If mixed light is used for exposure, i.e., light which contains also wavelengths from the fundamental lattice absorption region, a colored photoconductive layer is decolorized at the boundaries between the light-struck and non light-struck areas, i.e., the layer is changed in accordance with the image at the points where there is a sharp decay in the conductivity of the photoconductive layer. This effect, which permits the use of unfiltered light of incandescent lamps, i.e., light which also contains UV light, is utilized in one embodiment of the invention. This embodiment is illustrated schematically in FIG. 5 which is explained in detail below. In this case, a so-called auxiliary image is projected simultaneously with the original to be reproduced onto the photographic layer, preferably using the same source of light. Line or cross-line screens are, for example, suitable for use as auxiliary images. By illuminating and projecting the screen, sharp changes of the conductivity are produced in the exposed parts of the photoconductive layer; as explained above, in the absence of the auxiliary image, such conductivity jumps would only occur at the edges of the image. As already described above, the photographic layer is altered in accordance with the image at the points where these conductivity jumps occur. The entire exposed surface is thereby changed only in areas corresponding to the image. A

particularly uniform decolorization is obtained if care is taken to ensure that the conductivity jumps travel over the exposed areas; this can easily be achieved by moving the auxiliary image, i.e., the screen during exposure.

(1) PROCESS FOR THE PRODUCTION OF NEGATIVE IMAGES The simplest case consists in exposure to light which only contains wavelengths from the fundamental lattice absorption region, i.e., usually UV light. This possibility has already been described above under (1). After relatively long exposure times and simultaneous action of charge carriers, the dye at the unexposed areas of the photoconductive layer is changed, i.e., a negative image of the original is obtained.

This process can be greatly accelerated if simultaneously with the exposure to UV light the layer is exposed uniformly with light which contains substantially no rays with a wavelength from the fundamental lattice absorption region. This embodiment is shown schematically in FIG. 6 which is explained in detail below. Since the dye is more stable in those areas exposed to ultra violet light, uniform exposure of the photoconductive layer to the other light causes a visible change in the areas not exposed to ultra violet light in accordance with the image. A negative image of the original is obtained.

If light which contains wavelengths from the fundamental lattice absorption region is used for the uniform exposure of the photoconductive layer, this exposure must be performed through an auxiliary screen image. As auxiliary images are suitable screens such as line or crossline screens as described above. The conductivity jumps explained above are produced and again a visible change is produced in those areas of the photoconductive layer not exposed by the imagewise exposure with ultra violet light, i.e., a negative image of the original is obtained. In this case, the expression uniform exposure is, of course, no longer strictly correct since the light has passed through a screen. However, the expression uniform exposure is intended to mean that the exposure is with light not passed through the original image to be reproduced. Negative images are also obtained if the light passed through the original for exposure of the image is a mixed light containing also wavelengths outside the fundamental lattice absorption range. In this case, uniform exposure is also carried out in the above manner, with or without an auxiliary image.

According to a preferred embodiment of the invention the light which essentially consists of rays with wavelengths outside the fundamental lattice absorption region and which is used for the imagewise exposure in the production of positive images and for the non-imagewise exposure for the production of negative images, is substantially selected from those regions of the spectrum, absorbed by the dye of the photoconductive layer. More particular the dyes which are incorporated into the photoconductive layer areselected from those which are capable of optically sensitizing the photoconductive compound dispersed in the photoconductive layer. In this case the layer is exposed with light of a wavelength range in which the dye sensitizes.

Suitable photoconductive materials comprise all those which are known from the electrophotographic art. Such materials consist of an electrically conductive support and a photoconductive layer coated thereon. The photoconductive layers essentially consist of a photoconductive compound finely distributed in an insulating binding agent.

As photoconductors can be used organic as Well as inorganic compounds such as zinc oxide, zinc sulfide, cadmium sulfide, mercuric sulfide, lead iodide and the like.

Suitable binding agents include organic silicone resins, polyester resins, such as alkyd resins e.g., styrene-alkyd resins, silicone-alkyd resins, and oil-modified alkyd resins such as soya-alkyd resins, Useful resins are described in the book Chemie und Technologie der Kunststoffe by R. Houwink/A. J. Staverman in particular volume II, 2, published by Akademische Verlagsgesellschaft, Leipzig, 1963. As binding agents can further be used styrene-butadiene copolymers, polyvinylchloride, polyvinylacetate and the like. Suitable as supports are, for example, metal foils or metallized plastic foils, or paper of sufiiciently high electric conductivity.

The concentration of the dye in the photoconductive layer can vary within wide limits. The particular quantity depends on the effects desired. I have found that generally from about mg. to about 10 g. of dye per square meter of the photoconductive layer are quite adequate to obtain the desired results.

While it is preferred to incorporate the dye in dissolved from, the dye can also be heterogeneously distributed witin the photoconductivelayer.

Suitable are, for example, the following dyes:

Rose Bengale (Cl. 45,440)

Bromophenol Blue (British Patent 874,133) Eosin (C.I. 45,380)

Erythrosin (C.I. 45,430) Diamantfuchsinrot (Cl. 52,510) Alizarinbrilliantblau (Cl. 63,010) Methylenblue (C.I. 52,015)

With the process according to the invention, various dyes, in particular sensitizing dyes can also be employed in admixture with one another. If sensitizing dyes are used which are, for example, colored in the three primary colors yellow, magenta and cyan, it becomes possible to produce electrophotographic images in those colors.

If different dyes preferably sensitizing are used, the dyes can be dissolved together and the photoconductive substance, for example, zinc oxide, can be treated with this solution. For the production of multicolor images, it is preferred to dissolve the dyes separately and to mix the solutions separately with the photocondctive compound preferably zinc oxide. In this way, differently sensitized or colored zinc oxide types are obtained, which can either be used in mixed form in one layer or separately in different layers.

In many cases, it is sufficient to treat the photoconductive layer subsequently with a solution of the dye.

Suitable light sources are, for example, incandescent lamps, Xenon lamps and mercury vapour lamps. If, for example, a tungsten lamp is used as source of light then the undesired ultra violet light can be filtered out for the production of a positive image on a dyed photoconductive zinc oxide layer by interposing in front of the source of light a filter transparent only for light of wavelengths greater than about 400 millimicrons. It is advantageous to use filters which are particularly transparent in the absorption or sensitizing region of the dye, provided the filters absorb the wavelengths which correspond to the fundamental lattice absorption region of the photoconductive compound. If, for example, Rose Bengal is used as a dye in combination with zinc oxide as photoconductor, it is sufiicient if the filter is transparent for light of the region of approximately 480 to 600 millimicrons or only a part of this region. In this case it would be advantageous if the filter is transparent for light of the absorption maximum of about 564 millimicron of the dye.

The quantity of ultra violet light emitted by incandescent lamps can be reduced by operating the lamp at reduced radiation temperature. This can be achieved, e.g., by not applying the full operating voltage to the lamp. Incandescent lamps having filament temperature of between 1700 and 2200 C., preferably about 2000" C., are particularly suitable. For the production of negative images it is necessary to use for the imagewise exposure, a lamp which emits light from the fundamental lattice absorption region of the photoconductive compound. Thus, in the case of ZnO it would be necessary to use a source of light which emits at least partially in the near UV region. For this purpose there may be used, for example, a mercury vapour lamp in combination with a filter which is transparent only for ultra violet light, but incandescent lamps operated without filter at full operating voltage can be employed too. To accelerate the color change in the dark areas in the process described under (2) above, the photoconductive layer may be uniformly exposed to light from the absorption or sensitizing range of the dye in addition to the imagewise exposure with ultra violet light. In the case of Rose Bengal, the wavelengths for this uniform exposure would be between about 480 and 600 millimicrons.

In general, satisfactory results are obtained if the photoconductive layer faces the light source. If desired, exposure of the layer to incandescent light, i.e., light not containing ultra violet light can also be carried out from the backside.

The invention will be further described and explained in the accompanying schematic drawings:

FIGURE 1 shows a photoconductive material consisting of the photoconductive layer 1 containing the photoconductive compound in effective contact with a dye, preferably a sensitizing dye, arranged on an electrically conductive metal layer 2 which is applied to a plastic foil 3. The process of the present invention is carried out by imagewise exposing the photoconductive layer in accordance with the image to be reproduced. The light 4 used for the exposure is selected from that region of the spectrum which corresponds to the sensitization range of the dye. Simultaneosuly, the layer is subjected to a corona discharge by means of electrode 5, this being effected by moving the electrode several times over the layer. As a result, the sensitizing dye is bleached at the exposed areas, or changes at those areas into another color. In order to judge the rate of color-changing, the layer can be observed. The corona discharging is switched oif during this observation period.

FIGURE 2 illustrates an electrophotographic material consisting of a photoconductive layer 1 arranged on a transparent metal layer 6 which is carried by a transparent plastic support 7. In this case, the process is carried out by imagewise exposing the layer from the rear. The process is otherwise performed as described under FIG. 1. According to FIGURE 3, the process is carried out by imagewise exposing the layer through the original 8 to be reproduced by placing the original, for example, a sheet of typewriting paper bearing characters on one side, on the front of the layer, the light source and also the discharge electrode being on the same side as the layer surface.

According to FIGURE 4, the process is carried out in exactly the same way as illustrated in FIGURE 3, except that the original document 8 is on the back and is exposed to light from the back of the layer.

According to FIGURE 5, the process for the production of positive images may, for example, be carried out by using an incandescent light projector 11, for the projection of the original to be reproduced, in the instant case the diapositive 12 onto the surface of the dyed photoconductive layer 13 coated on the metal side of a metallized sheet of a synthetic polymer 14. Simultaneously with the diapositive 12, a line screen 15 is also projected on the surface of the photoconductive layer. This line screen 15 is moved back and forth. Simultaneously the photoconductive layer is subjected to corona discharge by means of the electrode 16. The dye changes color at the exposed areas. A positive image of the original is thus obtained.

According to FIGURE 6, the process for the production of negative image is performed, for example, by projecting a diapositive 12 onto the surface of a dyed photoconductive layer 13 coated on the metal side of a metallized support 14 of a polymeric material by means of a projector 17 which transmits light in the wavelength region of the fundamental lattice absorption of the photoconductive compound in the layer. At the same time, the entire layer 13 is illuminated by means of a second projector 18 which contains light in the wavelength region of the absorption of the dye but not in the wavelength region of the fundamental lattice absorption of the photoconductive compound. Simultaneously the layer is subjected to a corona discharging by means of the discharge electrode 16. The dye undergoes a change only at the nonimagewise exposed areas.

A negative image is obtained.

EXAMPLE 1 9 g. of a photoconductive zinc oxide and mg. of Rose Bengale dissolved in ethyl alcohol and are thoroughly mixed and dried. The zinc oxide containing dye is then ground with the solution of 0.75 g. of a phenylmethyl polysiloxane resin in 10.5 g. of toluene in a ball mill for one hour. The resulting mixture is cast onto the metal side of a metallized cellulose acetate support and dried. An image is projected onto the photoconductive layer with the aid of a 300 watt incandescent lamp. The layer is subjected to a corona discharge by means of a spherical platinum electrode having a diameter of about 1 mm. The distance between the layer and the electrode is about 11 mm. The potential between the corona electrode and the layer is 11 to 12 kv. If the intensity of the light-source is varied the following results are obtained:

(a) The full operating voltage of 220 volts is supplied to the lamp of the projector. This corresponds to an incandescent filament temperature of about 2500 C. This temperature is measured with a pyrometer. After a treatment time of several minutes, only the edges between the exposed and unexposed areas are bleached.

(b) A reduce-d voltage of about 120 VOlts is supplied to H the lamp of the projector. This corresponds to an incandescent filament temperature of about 2000 C. After 1.5 minutes, the exposed areas of the image are completely bleached.

(c) The full operating voltage of 220 volts is applied to the lamp of the projector. In front of this lamp is arranged a filter which is transparent only to light of wavelength greater than 480 millimicrons. After a treatment time of 1.5 minutes, the light-struck areas are bleached in accordance with the image.

(d) The experimental conditions are the same as in Example lb or 10. Superimposed on the incandescent light image, a UV light image is projected by means of a mercury vapor lamp. In front of the mercury vapor lamp is a filter which transmits only wavelengths in the region of 300 to 400 millimicrons. The dye is not bleached in the areas struck by the UV light.

Similar results are obtained if, for example, photoconductive rutile is used instead of zinc oxide, and Bromophenol blue, Diamond Fuchsin red or eosin instead of Rose Bengal. Experiments with layers which contain only the photoconductive compound without any binding agent led to similar results. This indicates that the type of binding agent used is of relatively minor significance.

EXAMPLE 2 The photoconductive element of Example 1 is processed as described in Example 1a. Sample of the photoconductive material are exposed with different lights which vary slightly in composition. This is accomplished by placing different interference filters all having about the same transmittance successively in front of the projec-- tor lamp. The maximum transmittance of the various filters is in the region of the wavelengths M2442; A =744; x =500; M=540; A =564; A =586; A =605; A :654; A 675 millimicrons.

The samples of the photoconductive layer are bleached by exposure through the original and each filter in turn, using otherwise the same conditions as in Example 1a.

Nine different results are thus obtained. The best bleaching is obtained that mean the best images are produced with the use of the filters 4, 5 and 6. In the maximum transmission range of these filters lies the maximum absorption of Rose Bengal. Only a slight bleaching effect is obtained at x:442 and no bleaching effect at A:654 and at \=675 millimicrons. In those regions, the absorption of Rose Bengale is also negligible (I. A. Amick, R.C.A. Review, vo ume XX (1959) page 768, FIGURE 11). It is readily apparent that it wavelengths of different spectral regions are used, the dye bleaches most rapidly in the spectral region of its maximum absorption.

EXAMPLE 3 A photoconductive layer containing zinc oxide and Rose Bengale as described in Example 1 is used. An image is projected onto the grounded photoconductive layer with ultra violet light from a mercury vapour lamp. A filter which only transmits light in the wavelength region between 300 and 400 millimicrons is interposed between the light-source and the photoconductive layer to be exposed. The corona discharge is performed by means of a spherical Pt electrode having a diameter of about 1 mm. The distance between the photoconductive layer and the electrode i 11 mm. The potential between the corona electrode and the layer is 11 to 12 kv. The dark areas of the image are bleached after a treatment time of about 6 to 10 minutes. A negative image of the original is obtained. Longer times are thus required for producing the negative image than for producing positive images.

EXAMPLE 4 The same arrangement as in Example 3 is employed but in addition the layer is at the same time uniformly exposed to light from the absorption range of Rose Bengale, having wavelengths greater than 480 ru The negative image is produced in 1 to 2 minutes, i.e., within a much shorter time than in Example 3, and is of better quality.

EXAMPLE 5 A layer of the same type as in Example 1 is used. The arrangement is otherwise as in Example 1a except that in addition a line screen, which is moved back and forth, is projected onto the layer. The areas of the photoconductive layer which are imagewise exposed are bleached after a treatment time of 1.5 to 2 minutes.

EXAMPLE 6 A photoconductive layer containing ZnO as photoconductor and Rose Bengale as dye is used. A circular disc is projected onto the layer with the aid of a 300 watt incandescent lamp. Various interference filters are placed successively between the light-source and the photoconductive layer. The corona discharge is accomplished by means of an electrode arranged at a distance of 11 mm. from the photoconductive layer. The potential between the layer and the electrode is 12 kv. Bleaching of the photoconductive layer to produce a positve image begins at Wavelengths of about 400 millimicrons and higher.

If a layer which contains indium-(III)-oxide as photoconductive compound is exposed under the same conditions, then the reversal point at which negative bleaching changes into positive bleaching lies at about 440 to 470 millimicrons.

This experiment shows that the reversal point at which positive bleaching changes into negative bleaching depends on the photoconductive compound in the layer. The gap between a valence band and the conduction band of zinc oxide corresponds to an energy of about 3.2 ev. which corresponds to a wavelength of 385 millimicrons. The same data of In O is about 2.8 ev. which corresponds to a wavelength of about 440 millimicrons. Thus, both in the case of ZnO and in the case of In O the reversal point of negative bleaching to positive bleaching is situated at wavelengths corresponding approximately to the gap between valence and the conduction band. The

smaller that gap the longer will be the wavelength at which the reversal from negative to positive bleaching takes place.

I claim:

1. A process for the production of a direct image by exposing to an original image a photoconductive element comprising a dyed photoconductive layer containing a photoconductive compound in effective contact with a dye that absorbs light of wavelength outside the fundamental lattice absorption region of the photoconductive compound, said exposure being with mixed light containing wavelengths which correspond to the fundamental lattice absorption region as well as wavelengths outside said region but within the absorption range of the dye, simultaneously producing sharp changes in conductivity in the imagewise exposed areas of the photoconductive layer, and simultaneously subjecting the photoconductive layer during the imagewise exposure to a corona discharge to cause the dye to change color at the exposed areas and produce a direct positive visible dye image of the original.

2. A process as defined in claim 1 wherein the exposure is performed with light having a wavelength in the maximum absorption range of the dye dispersed in the photoconductive layer.

3. A process as defined in claim 1 wherein the sharp changes in conductivity are produced by imagewise exposing the photoconductive layer through a screen.

4. A process as defined in claim 3 in which the image of the screen is moved during the exposure to the screen.

5. A process for the production of a direct image by imagewise exposing to an original a photoconductive element comprising a dyed photoconductive layer which contains a photoconductive compound in effective contact with a dye that absorbs light of wavelength outside the fundamental lattice absorption region of the photoconductive compound, said exposing being with light having wavelengths which correspond to said fundamental lattice absorption region, simultaneously uniformly exposing the photoconductive layer to light absorbed by the dye and consisting essentially of wavelengths outside said fundamental lattice absorption range, and simultaneously also subjecting the photoconductive layer to a corona discharge to cause the dye to change color at those areas which are not exposed by the imagewise exposure, to form a negative visible dye image of the original.

6. A process as defined in claim 5 wherein the uniform exposure is performed with light having a wavelength which corresponds to the maximum absorption range of the dye dispersed in the photoconductive layer.

7. A process according to claim 5 wherein the dye is selected from the group consisting of Rose Bengal (Color Index 45,440), Bromophenol Blue, Eosin, Erythosin (Color Index 45,430), Diamantfuchsinrot (Color Index 52,510), Alizarinbrilliantblau (Color Index 63,010) and Methylene Blue, and the photoconductive compound is selected from the group consisting of zinc oxide, zinc sulfide, cadmium sulfide, mercuric sulfide, lead iodide and indium-(III)-oxide.

8. A process for the production of a direct image by imagewise exposing to an original a photoconductive element comprising a dyed photoconductive layer which contains a photoconductive compound in eifective contact with a dye that absorbs light of wavelength outside the fundamental lattice absorption region of the photoconductive compound, said exposing being with light having wavelengths which correspond to said fundamental lattice absorption region, simultaneously uniformly exposing the photoconductive layer to light absorbed by the dye and having wavelengths both within and without said fundamental lattice absorption region, the uniform exposure being an imagewise exposure to a screen, and simultaneously also subjecting the photoconductive layer to a corona discharge to cause the dye to change color at those areas which are not exposed by the imagewise exposure, to form a negative visible dye image of the original.

9. A process for the production of a direct positive image comprising (1) imagewise exposing a photoconductive element comprising a photoconductive compound in effective contact with a dye that absorbs light of wavelength outside the fundamental lattice absorption region of the photoconductive compound, said exposing being with light consisting essentially of wavelengths lying outside of the fundamental lattice absorption region of the photoconductive compound but including wavelengths absorbed by the dye, while (2) simultaneously subjecting the photoconductive layer to corona discharge, to cause the dye to change color at the exposed areas.

10. A process according to claim 9 wherein the dye is selected from the group consisting of Rose Bengal (Color Index 45,440), Bromophenol Blue, Eosin, Erythrosin (Color Index 45,430), Diamantfuchsinrot (Color Index 52,510), Alizarinbrilliantblau (Color Index 63,010) and Methylene Blue, and the photoconductive compound is selected from the group consisting of zinc oxide, Zinc sulfide, cadmium sulfide, mercuric sulfide, lead iodide and indium- (III) -oxide.

11. A process for the production of a direct positive image comprising (1) imagewise exposing a photoconductive element comprising a photoconductive compound in effective contact with a dye that absorbs light of a wavelength outside and the fundamental lattice absorption region of the photoconductive compound, said exposure being both to light within said fundamental lattice absorption region as well as to light outside said region but within the absonption range of the dye, v(2) while simultaneously producing abrupt changes in conductivity in the exposed areas of the photoconductive layer by interposing a moving screen between the light source and the exposed areas of the photoconductive layer, and (3) simultaneously subjecting the photoconductive layer during said imagewise exposure to corona discharge, to cause the dye to change color at the exposed areas.

References Cited UNITED STATES PATENTS 9/1962 Greig 961.7 3/1963 Miller et al. '96-l I. TRAVIS BROWN, Primary Examiner.

C. E. VAN HORN, Assistant Examiner. 

